eso9214 — Science Release

New Planet Found in the Outer Solar System

2 October 1992

A new planet has just been found in the outer solar system. Although the observations do not yet allow an accurate determination of its orbit, it appears that it is situated about 6,000 million km away, outside the orbit of the outermost, known planet, Pluto. No other object has ever been found this far out in the solar system.

Discovery and Follow-up Observations

The new planet, which has received the provisional designation 1992 QB1, was first seen by American astronomers David Jewitt and Janet Luu, working with the University of Hawaii 2.2-metre telescope at Mauna Kea on the Big Island of Hawaii. They noticed the faint object on August 30, 1992 and again on the two following nights, in the constellation of Pisces (The Fish), rather near the celestial equator. The image was star-like and the motion was very slow, indicating that the object might be a very distant minor planet. The brightness of 1992 QB1 was about 23, or 6,000,000 times fainter than what can be seen with the unaided eye. The colour was reddish.

An excellent extrapolation of the motion by Brian Marsden of the Minor Planet Centre of the International Astronomical Union (Cambridge, Mass., USA) permitted renewed observations of 1992 QB1 after the full-moon period in mid-September. At the ESO La Silla Observatory, astronomers Alain Smette (ESO) and Christian Vanderriest (Observatoire de Meudon, France) obtained three images with the 3.5 m New Technology Telescope on September 27 and 28. Reproductions of these images are shown on the photo accompanying this Press Release. The brightness was about the same as at the time of the first observations, one month earlier.

These images were transmitted via the permanent satellite data link to the ESO Headquarters in Garching, where astronomer Olivier Hainaut (ESO) accurately measured the positions of the slowly moving planet. With the help of the new positions, Brian Marsden was able to confirm the great distance of 1992 QBl, but due to its extremely slow motion, it must be observed during another couple of months, before a more accurate orbit can be computed.

The Nature of 1992 QB1

On the assumption that it moves in a circular orbit around the Sun, Brian Marsden estimates its distance from the Sun to be about 41 Astronomical Units (AU), or more than 6,000 million km, i.e., just outside the orbit of the most distant known planet Pluto. This corresponds to a period of revolution around the Sun of 262 years. Pluto was discovered in 1930 by Clyde Tombaugh at the Lowell Observatory in Arizona, USA, and no other object that orbit the Sun has ever been found at such a large distance .

From its brightness, the diameter of 1992 QBl is estimated at 200 km. It therefore belongs to the class of Minor Planets. However, almost all Minor Planets move inside the orbit of the giant planet Jupiter (5.3 AU, or only 800 million km from the Sun), and only two are known which are at the distance of Saturn.

It cannot at this moment be entirely excluded that the new object is an extremely distant comet that has undergone an outburst, similar to that of Comet Halley in early 1991 which was extensively observed at ESO. However, the constant brightness during one month does not support this speculation and astronomers believe that it is a planet with a solid surface, probably covered with a reddish residue of organic-rich materials.

1992 QBl is an extremely interesting object and will be intensively observed with large telescopes during the coming months, at ESO as well as at a few other observatories.